Sayan Maity

• The light-harvesting protein-pigment complexes of plants, bacteria and algae play a major role in the conversion of solar energy into sustainable forms of chemical energy during photosynthesis. Chlorophyll, bacterio-chlorophyll and bilin molecules are the key pigments present in those complexes which mainly control the excitation energy transfer processes. However, due to the large size and the multiscale quantum-classical methods are often required to study the dynamical processes within protein environments. In this direction, the present thesis aims to formulate an efficient strategy to describe the underlying foundation of energy transfer process in these complexes and provides an outlook for the modeling of artificial LH complexes. To this end, the density functional based tight-binding (DFTB) method has been applied to perform ground state molecular dynamics simulations coupled to a classical environment within a quantum mechanics/molecular mechanics (QM/MM) fashion. In a subsequent step, the time-dependent extension of the long-range-corrected DFTB formalism has been applied to calculate the excitation energies of each pigment molecules also in a QM/MM setting. This provides the basis of the multiscale scheme which is then applied to various bacterial and plant LH complexes to extract the major excitonic parameters like site energies, couplings and spectral densities. Moreover, based on these input parameters, exciton dynamics and transfer rate calculations have been performed for some of these systems. Furthermore, the calculated results were compared with the experimental counterparts resulting remarkable agreement for these large bio-molecular systems. These findings motivated further investigations on an artificially designed light-harvesting system by employing porphyrin molecules attached to a clay surface which is experimentally found to exhibit excellent energy transfer properties.